IMSI | Area and Subscriber Identities

The IMSI allows unambiguous identification of a particular SIM or USIM card. The IMSI is composed of three parts (Figure 1):

• The Mobile Country Code (MCC), consisting of three digits. The MCC uniquely identifies the country of domicile of the mobile subscriber. MCC values are administrated and allocated by an international numbering plan.
• The Mobile Network Code (MNC), consisting of two or three digits for GSM/UMTS applications. The MNC identifies the home PLMN of the mobile subscriber. The length of the MNC (two or three digits) depends on the value of the MCC. A mixture of two- and three-digit MNC codes within a single MCC area is not recommended and is beyond the scope of this specification.
• The Mobile Subscriber Identification Number (MSIN), identifying the mobile subscriber within a PLMN. As a rule the first two or three digits of the MSIN reveal the identity of the Home Location Register (HLR) or HSS that is used for Signaling Connection Control Part (SCCP) Global Title translation procedures when roaming subscribers register in foreign networks.
  
  

 

Figure 1: Structure of IMSI (according to 3GPP 23.303). Reproduced with permission from © 3GPP™

The National Mobile Subscriber Identity (NMSI) consists of the MNC and the MSIN.

A combination of MCC and MNC can be used to aggregate call-specific performance measurement data (such as cumulative counters) on IMSI groups. This will help to highlight problems of roaming subscribers such as network failures during registration procedures, as described later in this book. Table 1 shows some samples from an IMSI group mapping table with MCC/MNC combinations in "IMSINumber" fields and operator names in the "IMSIGroupName" field. Note the three-digit MNC used for the American operator.

Table 1: IMSI group mapping table from Tektronix Communications NSA software

<IMSI IMSINumber= ‘26201’ IMSIGroupName= ‘T-MOBILE DEUTSCHLAND GMBH (GERMANY)’ />

<IMSI IMSINumber= ‘26202’ IMSIGroupName= ‘VODAFONE D2 GMBH (GERMANY)’ />

<IMSI IMSINumber= ‘26801’ IMSIGroupName= ‘VODAFONE TELECEL (PORTUGAL)’ />

<IMSI IMSINumber= ‘27201’ IMSIGroupName= ‘VODAFONE IRELAND PLC (IRELAND)’ />

<IMSI IMSINumber= ‘310560’ IMSIGroupName= ‘T-MOBILE USA, INC. (UNITED STATES)’ />

It is possible that one-use equipment will work with more than just one (U)SIM. A good example is a mobile phone that has both business and private SIM cards as one device. Depending on the nature of the call (private or business), the owner of the handset can choose which (U)SIM should be used to make the call. Such a procedure might be required in case private phone calls need to be charged separately due to national income tax laws (as found, for example, in Germany).